Haste above or between those breakpoints still has value (lower cast times, shorter GCD, etc), just not as drastic a gain as when u reach a break point. Haste always adds to HPS, but only adds to HPM at break points

There is no cap on mastery: more will always heal more...and somehow i doubt you will be at 100% crit chance anytime soon. Crit and Mastery both increase HPM (when not overhealing) and HPS (though less than haste)

in general, just go by the stat weights INT >> Spirit (till u no longer need regen) / Haste >> Mastery / Crit (the more u focus on tank healing, the better mastery is than crit)

Your spec is... interesting? At first glance it looks like a PvP tree, but then the lack of Furor and Revitalize is bewildering. It looks like you were determined to buff Rejuv and SM as much as possible without regard for regen.

As for gearing, assuming you consistently have a Lock who isn't an idiot, Dark Intent + 5% spell haste means you only need 1595 haste to get the 9th tick on WG. Which is huge for our raid healing right now.